In the terrorist radiation exposure scenario, radiation victims likely suffer from additional injuries such as infection and sepsis. Despite advances in our understanding of radiation injury or advances in the management of sepsis patients, little information is available regarding radiation combined injury (e.g. radiation exposure followed by infection and sepsis). In our preliminary studies, we have shown that radiation combined injury causes tissue damage, proinflammatory cytokine upregulation, and apoptosis. Radiation combined injury decreases the production of a novel gut-brain peptide ghrelin and increases the sympathetic activity. We discovered that administration of rat ghrelin after radiation combined injury produces various beneficial effects. Ghrelin is a 28-amino acid peptide in both humans and rats with difference in two amino acids. Ghrelin inhibits the sympathetic nervous system, as evidenced by the reduction of norepinephrine (NE) release and inactivation of sympathostimulatory neurons in the brain in sepsis. A specific ghrelin receptor antagonist increases norepinephrine and TNF-a production in normal animals and worsens sepsis-induced mortality. The action of ghrelin in sepsis also involves the parasympathetic nervous system, as vagotomy prevents its beneficial effects. Although administration of rat ghrelin after radiation combined injury is protective, it remains unknown whether human ghrelin is also beneficial. An obstacle hampering development of ghrelin as a therapeutic agent is the potential immunogenicity of rat peptides in humans. We therefore hypothesize that administration of human ghrelin attenuates tissue damage and inflammation, and reduces mortality after radiation combined injury. We further hypothesize that the beneficial effect of human ghrelin is mediated by its sympathoinhibitory and parasympathostimulatory properties. Thus, the goal of this R21/R33 project is to complete the preclinical development of human ghrelin as a novel therapeutic agent in reducing mortality after radiation combined injury and to determine the mechanism responsible for its beneficial effects. In the R21 Phase, we will 1) determine whether human ghrelin attenuates tissue damage and inflammation; 2) assess whether human ghrelin improves survival; and 3) conduct feasibility experiments demonstrating the effect of human ghrelin on the autonomic nervous system after radiation combined injury. In the R33 Phase, we will 4) conduct in-depth studies to delineate the mechanism responsible for human ghrelin's beneficial effects; 5) determine the dose-response effect of human ghrelin on apoptosis, cardiovascular response, tissue damage, and proinflammatory cytokines, as well as its dose- and time-course effect on survival after radiation combined injury; and 6) assess the toxicity and pharmacokinetic properties of human ghrelin in normal and injured animals. These proposed studies should provide useful mechanistic and preclinical information that will allow us to file an IND application to the FDA for initiating clinical trials in order to further develop human ghrelin as a safe and effective therapy for victims with radiation exposure and subsequent infection and sepsis. PUBLIC HEALTH RELEVANCE STATEMENT: In the terrorist radiation exposure scenario, radiation victims likely suffer from additional injuries such as infection and sepsis. Infection and sepsis after the initial radiation injury can significantly increase mortality. However, very little information is available regarding radiation combined injury (i.e., radiation exposure followed by infection and sepsis). It is obvious that there is an urgent unmet medical need for an effective and novel therapy for victims with radiation combined injury. In this project, we propose an innovative approach to develop a new medicine for the treatment of radiation combined injury.